This invention relates to producing modulated gradient fields in magnetic resonance imaging (MRI) systems.
In a typical MRI system, three electromagnetic gradient coils are driven using respective power supplies to generate magnetic field gradients in each of three orthogonal (e.g., x, y, and z) directions, termed "gradient coil directions." The magnetic fields generated by the gradient coils generally have components parallel to the direction of a main magnetic field, where the strengths of the components vary spatially (e.g., linearly) in each of the respective gradient coil directions. The gradient coils are typically driven to generate "read," "phase," and, "slice" gradient waveforms, where the plane or cross-section to be imaged lies parallel to the plane defined by the directions of the read and phase gradients.
A variety of gradient coil power supplies are known for generating magnetic field gradients for MRI systems. In some systems, the power supply consists only of an amplifier, which drives the gradient coil directly to produce the desired gradient waveforms. The amplifiers can be controlled to obtain images of planes that lie at oblique angles to the gradient coil directions.
In other MRI systems, such as those disclosed in Rzedzian U.S. Pat. No. 4,628,264, incorporated herein by reference in its entirety, the gradient coil is connected in parallel with a capacitor to form a parallel resonant circuit. And in still other MRI power supplies, such as those disclosed in Rzedzian et al. U.S. Pat. No. 5,285,161, incorporated herein by reference in its entirety, the gradient coil is connected in series with a capacitor to form a series resonant circuit. In both the parallel and the series resonant power supplies, an amplifier drives the coil-capacitor circuit at or near its resonant frequency, generating a sinusoidally varying gradient waveform. The power supplies can be controlled to interrupt the resonant sinusoidal waveform for periods of arbitrary duration.